If we're shipping the alfalfa to China, I assume that means it's supporting some Chinese person's food source, whether they are directly eating the alfalfa, or some animal is eating it that later becomes food.

If someone is flooding a field unproductively just to use up their quota of water, that is a bad thing that should be addressed. But even if you excluded that unproductive usage and compared AI water use to legitimate agriculture use, that would still be an unfair comparison. If you were to compare AI water use to the amount of water that people are wasting just for legal reasons, then I honestly think that would be a pretty apt comparison.

Pointing to agriculture as a necessity while also wanting water usage to be "productive" is a little contradictory here. We grow things because there is a demand for those products in similar way that there is a demand for datacenters, the nutrition aspect is secondary and has been for a long time now. Would you say that almond growing is a productive use of our water? How about bananas, or beef, or avocados? All of these products use an abnormally large amount of water compared to other agricultural endeavors and if we compare that to data center water usage data center's are a drop in the bucket. We don't 'need' all of products we produce through agriculture to survive anymore, we grow them because we like them.

Lots of Colorado river water goes to supplying year around lettuce. If we didn't have lettuce they would just eat something else. Given the supply constraints of the region, "but someone is eating it" is a really bizarre argument. It can be grown elsewhere without water problems.

The southwest is basically exporting its water very cheaply in the form of agriculture. Why when its such a constrained resource here?

What makes AI use "illegitimate", and any food use automatically "legitimate"?

People have all kinds of needs in addition to those for food and water.

They are pointing out that some locations are not a good place to grow specific things and that there is a lot of water wastage in doing so. Attempting to grow crops in the desert vs. in a temperate climate probably uses more water for the same amount of crops (unless they are desert plants, I guess). This is what's being pointed out. If I decide to grow tomatoes on the moon and then ship them back to Earth to be consumed, it's fair game for people to point out how much of a waste of resources that is vs. just growing them on Earth.